The following U.S. patents are believed to represent the current state of the art: U.S. Pat. Nos. 6,062,908; 6,116,963; 6,325,664; 6,176,741; 6,193,560; 6,224,425; 4,726,790; 4,729,743; 4,804,332; 4,929,196; 5,057,041; 5,112,253; 5,865,648; 5,397,250; 5,094,629; 5,102,354; 5,147,223; 5,151,054; 5,158,482; 5,213,522; 5,224,878; 5,266,054; 5,286,221; 5,344,342. 6,473,608
The disclosures of all publications mentioned in the specification and of the publications cited therein are hereby incorporated by reference.